Once in a prior life I was on a long telecommunications survey project to Fort Sam Houston, in San Antonio, TX. In July. 90+F plus 90% humidity.

Where if you laid your manhole cover puller (kind of a crowbar) down in the sun, you'd better be wearing gloves when you picked it up, or you'd blister your hand. Where according to the heat and humidity tables, you were only supposed to work about 20 minutes before taking a break in the shade.

Anyway, the base safety officer gave us a safety briefing required of all workers about working in the heat. She told us something that always stuck with me: you can easily gauge your hydration levels by the color of your pee (and indeed whether or not you are even peeing).

She said, when you observe the color of your urine, you should think 7-Up, not Mountain Dew. Your pee should be more on the clear side rather than a heavy, yellow color.

If your pee is too dark (or being produced in very limited quantities) you need to be drinking water. Indeed, when you know conditions are such that you will be sweating, you need to be drinking well before you even get thirsty.

I have consistently applied this rule of thumb to Ultrarunning. When Nature calls along the trail and you pull off for a bio-break, observe the color of your urine. It may keep you out of serious trouble.

This observation comes from the the school of hard knocks. Once in a road race I was trying to PR on an ugly hot and humid day. I didn't scale back my expectations but instead doubled down and ran even harder. That worked OK for about 5 miles...then I remember suddenly feeling disoriented, unable to run straight, then nothing. I woke up in an ambulance and had to be treated at the hospital for heat exhaustion. So it's a big deal.